1. Field of the Invention
The present invention relates generally to reference circuits and, in particular, to bandgap reference circuits that provide reference signals of substantially constant voltage levels.
2. State of the Art
Many electrical devices have a reference circuit for generating a reference signal for internal use that is based or derived from an external source. The external source is often a supply voltage with the generated reference signal being representative of either a reference current or a reference voltage. The reference circuit is usually designed such that the reference signal maintains a constant level over variations in the supply voltage, over a range of temperatures, and over manufacturing process variations.
One form of a reference circuit is known as a bandgap reference circuit. Bandgap reference circuits are well known in the art of analog integrated circuit (IC) design for generating a reference voltage equal to the electron bandgap level of silicon devices, which is approximately 1.2 volts. Bandgap reference circuits generally provide precise reference signals.
A conventional bandgap reference circuit utilizes bipolar transistors to provide the bandgap function. When complementary metal oxide semiconductor (CMOS) devices are implemented, the bandgap reference circuit generally utilizes parasitic bipolar transistors. A conventional bandgap circuit relies on the difference of the base-emitter junction voltages to provide a linear temperature correction voltage which is proportional to the absolute temperature (PTAT). Additionally, the base-emitter junction voltage VBE is proportional to the negative coefficient of temperature (i.e., the VBE measurement is used to track and correct changes in the reference circuit caused by temperature variations). The combination of these two effects results in the bandgap reference signal exhibiting a near-zero temperature coefficient which allows devices that utilize a bandgap reference circuit to operate with a reference signal that exhibits high accuracy.
Conventional bandgap reference circuits are known to have two stable operating states only one of which is entered when an external supply source is applied to the reference circuit during a power up condition. The first operating state corresponds to a desired operating state wherein the reference circuit supplies or generates the desired reference signal. The second operating state corresponds to an undesired state of the circuit in which the referenced circuit remains in a shutdown or inoperative condition wherein no reference signal is generated. One shortcoming of conventional bandgap reference circuits is that once the circuit enters the undesired state, the circuit tends to remain locked-up in the undesired state for an indeterminate period of time before transitioning in response to significant external stimulus, if transitioning is at all possible, to the desired operating state.
One approach for avoiding start up problems associated with bandgap reference circuits is to incorporate a start-up circuit that ensures that the bandgap reference circuit initializes to the desired operating state. One shortcoming with conventional start-up circuits is that they have been designed for responding to external source or supply voltage levels greater than approximately 1.5 volts. In many conventional electrical devices, such a supply voltage level is available and therefore sufficient such that conventional start-up circuits utilized in bandgap reference circuit designs are adequate. However, in devices where a reduced supply voltage is preferable, generating a reference signal using conventional higher supply voltage circuits becomes difficult. Accordingly, it would be desirable to provide a reference circuit that overcomes these and other drawbacks of the prior art. More specifically, it would be desirable to provide a reference circuit that can operate at power supply voltage ranges below 1.5 volts.